Subsurface wastewater disposal systems, commonly called septic tank systems or septic systems, are widely used for on-site processing of wastewater from dwellings and other smaller volume wastewater sources. Typically, wastewater is delivered via a wastewater line to a septic tank for primary processing. The septic tank effluent, or wastewater, is flowed to a leaching system for secondary processing by means of distribution pipes. The leaching system, also commonly called a disposal field, leach field, or infiltration field, typically comprises permeable soil of the earth and some sort of excavation in the soil which is filled with stone particulate such as crushed stone or coarse gravel (typically 2.5 cm in dimension) and or a mechanical component, the function of which is to convey wastewater through a conduit, to infiltrate it into the soil.
The principal function of the septic tank is to effect primary wastewater processing by engendering physical separation and retention of solids which are lighter and heavier than water, typically by settling and baffling. Solids, which settle out as sludge, are mostly decomposed by action of bacteria in a typically anaerobic environment. Gases, which are generated in the process, are vented to atmosphere. The wastewater from the septic tank is typically conveyed to the leach field by passing it through a distribution box and piping which channels wastewater to the leach field trenches, in a predetermined fashion. The wastewater is supposed to be free of solids of significant size. It will contain suspended solids of fine size, microorganisms such as bacterium and viruses, and various chemical constituents.
The purpose of the leach field is generally to cause the wastewater to be treated or renovated, so it can be benignly returned to the hydrologic cycle that characterizes the movement of water into, through, and from soil beneath the surface of the earth. What follows is a simplified version of certain conventional ways of looking at leach field operation phenomena, to provide a conceptual framework for appreciating the invention. It is not intended to be comprehensive nor limiting.
As the wastewater travels from within a leach trench and through the soil in a properly functioning system, it is subjected to natural chemical and biological processes within a “zone of influence”, which may extend 30-120 cm from the trench interface with the soil. A traditional leach field is comprised of a trench filled with small (2-3 cm) stone pieces. A perforated pipe runs through the stone, delivering the wastewater along the trench. A popular modern type of leach field comprises a series of interconnected arch shaped molded plastic chambers having perforated walls, such as leaching chambers sold under the Infiltrator brand name. See U.S. Pat. No. 5,401,116 of J. Nichols, and U.S. Pat. No. 5,511,903 of J. Nichols et al. Typically, Infiltrator® chambers are directly buried in a trench in substitution of the stone-and-pipe leaching device.
The leach field must have sufficient capacity to receive and properly process the anticipated flow of wastewater. The steady state capacity, or the infiltration rate, of a leach field is a function of the resistance to wastewater flow of the surfaces of the trench and the surrounding soil, as such may be influenced by hydraulic phenomena other than permeability, such as capillary action. For illustration here, only the sidewall of the trench will be now discussed. If distilled water is processed in sterile soil of a leach field, the infiltration rate is purely a function of the mechanics and hydraulics of the soil. However, in that wastewater contains organic substances, over time, an active, stable, moist biological crust layer frequently grows on surfaces. Of particular interest is the crust layer which occurs on a trench sidewall and within the nearby soil, especially when the layer tends to block openings in leaching system conduits.
The crust, also commonly called a biomat or biocrust, is an organic layer, typically 0.5-3 cm thick. It is normally less permeable than the surrounding soil. Thus, the biomat often significantly determines the long-term steady state infiltration capacity of a leach field. The biomat also serves as a filter for bacteria and some suspended solids. In a properly functioning system, the surrounding soil to remain desirably unsaturated and aerobic, thus enabling antibiotic attack of any pathogenic bacteria, and more importantly, chemical reactions involving free oxygen. Biomat is thought to aid in filtering things which enter the influence zone. Nitrogen, discharged in human waste, is characteristically passed through any biomat, predominantly as ammonium (NH4+), to be nitrified, or converted to nitrate (NO3) form, in the aerobic environment of the influence zone and adjacent soil. Foreign constituents in the wastewater may also sorb and or react with soil constituents; or they may ultimately be only diluted upon return to the ground water. As the wastewater is renovated in the influence zone, it moves mostly outwardly and downwardly toward the ambient water table in the earth. Some water may move upwardly into the vadose above the trench, by capillarity, evaporative-uptake and plant-uptake. It is usually required that the bottom of the leach field trench be a particular distance above the ambient water table, because sub-optimal wastewater treatment conditions exist in the extremely moist soil, the capillary fringe, just above the water table.
In a properly designed, used and maintained septic tank disposal system, once biochemical equilibrium is reached, the capacity of the leach field remains stable insofar as infiltration or leaching capacity. A long term infiltration rate, or liquid acceptance rate, characteristic soils of southern New England, USA is about 8-32 liters/m2/day. However, too frequently, a septic tank system will demonstrate insufficient infiltration capacity. Typically, a failure is manifested by escape of wastewater to the surface of the soil, or by a substantial backing up of wastewater in the wastewater line. One cause of failure can be gross flow of solids from the septic tank into the leach field piping or chamber system, and blockage of the perforations in such components. The typical best remedy for such is to replace or extend the leach field. Failure can also be manifested by an inability of a given system to handle normal peak loads of wastewater which were handled in the past; and by inadequate purification of the wastewater in the influence zone, resulting in pollution of the groundwater. And, even if a system has not failed, it is desirable to guard against failure by having the greatest economically feasible margin of safety against failure.
Among the known causes of some failures are the following. The design of the system has become inadequate for the current conditions, either due to growth of a very heavy biomat, a changed character of wastewater, or changed conditions within the soil in the influence zone. For instance, the biological oxygen demand (BOD) of the wastewater may have been increased, or the ambient soil conditions changed, so that the desired biochemical conditions for stable aerobic function in the influence zone are no longer obtained. An accumulation of unreacted wastewater within the influence zone limits oxygen transport. Thus, a cascading type of failure mode may ensue, wherein the influence zone gets bigger and bigger as it gets less and less effective.
Cesspools, favored in some regions, avoid the use of septic tanks. Untreated wastewater from a source is dumped into and partially treated by natural processes in the pool of an underground pit; and, the the wastewater infiltrates into the influence zone of soil surrounding the pit for further treatment. Phenomena and problems similar to those described for leach fields will exist in cesspool influence zones.
Thus, there is a need for alternatives to the costly or sometimes physically impossible remedy of adding to or replacing the leaching system. And, if good technology is at hand, the possibility arises for putting in a smaller system initially and reducing cost, for providing greater margin of safety in any given system, or for allowing growth in use of an existing system.
Various approaches to enhance the capacity of leaching systems have been tried, reflecting different concepts of both failure and remedy. Chemical remedies in the forms of solvents, enzymes, and other proprietary formulations, for deposit into the wastewater line with wastewater, are commercially sold, but most are disdained or ignored by professionals. U.S. Pat. No. 5,588,777 of Laak discloses the injection of soap into the leach field. U.S. Pat. No. 5,597,264 of Laak discloses a method of periodically back flushing the leach field with water. U.S. Pat. No. 4,333,831 of Petzinger describes the type of problem mentioned above, solving it by using evaporation chambers in substitution of any leach field. U.S. Pat. No. 3,907,679 of Yost describes a system in which low pressure air is forced through a septic tank and then into a long coil of wastewater piping, so wastewater evaporates into the air and is discharged to atmosphere. U.S. Pat. No. 3,698,194 of Flynn describes how air is blown into a conduit of a leach field and vented from risers at the remote end of conduit, to cause evaporation of liquid in, and to dry out grease in, the conduit, during periods when the conduit is not being used for wastewater treatment. U.S. Pat. No. 4,013,559 of Johnson describes how air is introduced into the septic tank, flowed through unique vertical concrete panel leaching system units, and then discharged to atmosphere, to encourage aerobic conditions in wastewater within the panels. However, none of these prior art technologies seem to have found wide spread use. Thus, there is a continuing need for new ways to enhance the design and performance of leaching fields, both as they are originally installed and for when there are in need of rejuvenating.